From the infinitive 'to sible' (pronounced 'sibble'), meaning to perform some form of sexual act with another persons brother or sister (preferably younger for greater novelty value...though not TOO much younger). For clarity in use, the given example would mean that to sible 'her', would be to sleep with 'her' sibling.

'Dude, I would totally sible her...soon as her younger sister's street legal of course.'

1. The reason your mom knows where you hide your porn, drugs, and alcohol
2. Demonic forces sent down from wherever to punish those who try to enjoy their life
3. Punching bags
4. Someone to blame the broken window on.